This invention relates to the preparation and treatment of metal films and coatings, and in particular to a process for the deposition of a metal in a porous form. The invention also relates to the manufacture of electrolytic capacitors from such treated metal films or coatings.
Porous metal films are employed in a variety of industrial applications. For example, wrapped foil electrolytic capacitors are fabricated conventionally from aluminum foil anodes and cathodes that have been chemically or electrochemically etched to produce a high surface area. Typically a strip of aluminum foil is etched to provide a microporous surface which is then anodized to produce a uniform dielectric layer.
There are a number of problems involved in the etching process, which nevertheless is the standard commercial process for the production of sufficiently high capacitance foils. Etching requires the use of large scale aqueous solution treatment and coating baths and presents maintenance and effluent disposal problems. Also etching solutions typically contain ions such as a chloride (Cl.sup.-) which inhibit anodization and must therefore be thoroughly removed before anodization can take place. Furthermore the nature of the etched pits in the metal foil is such that high capacitance foils have narrow tunnels which results in a rapid fall in capacitance and a consequent deterioration in high frequency performance at higher anodizing voltages. This effect is caused by complete filling of the etched pore by the anodic oxide formed in the anodizing process.